Actually render emoji in calendar event titles; stack duplicate shared events

The earlier fix stripped emoji instead of rendering them, which wasn't
what was asked for. Event titles now draw with two fonts: the usual
default font for text, and a vendored monochrome emoji font (Noto
Emoji, OFL-1.1) for actual emoji runs, so they show up as real glyphs
instead of a tofu box or nothing at all. Monochrome rather than color,
since reliably rendering COLR/CBDT color glyphs depends on how Pillow's
FreeType was built -- not something to depend on across deployments.

Also: events sharing the exact same title and time across different
calendars (e.g. a shared family event synced onto more than one
person's calendar) now collapse into one row instead of showing twice,
with a color bar split between every contributing calendar so it's
still clear whose event it is.

server/app/calendar_feed.py

@@ -115,8 +115,18 @@ def merge_events(
     fetch_summary is "" when every source succeeded, else "N of M
     calendars unavailable" (never *which* source -- naming whose feed is
     down to everyone who looks at a shared household display is a bigger
-    overshare than the outage itself)."""
+    overshare than the outage itself).
+
+    Events sharing the exact same (summary, start, end, all_day) across
+    different calendars -- e.g. a shared family event synced onto more
+    than one person's calendar -- collapse into one entry rather than
+    showing as duplicate rows. Every merged event carries a "sources"
+    list ([{"owner_display_name", "color_index"}, ...], length 1 for an
+    ordinary non-duplicated event) that calendar_render.py draws a
+    color indicator per entry of, so a collapsed event still visibly
+    shows every calendar it came from."""
     merged: list[dict] = []
+    by_key: dict[tuple, dict] = {}
     failures = 0
     for source in sources:
         try:
@@ -130,9 +140,15 @@ def merge_events(
             failures += 1
             continue
         for event in events:
-            event["owner_display_name"] = source.owner_display_name
-            event["color_index"] = source.color_index
-            merged.append(event)
+            source_entry = {"owner_display_name": source.owner_display_name, "color_index": source.color_index}
+            key = (event["summary"], event["start"], event["end"], event["all_day"])
+            existing = by_key.get(key)
+            if existing is None:
+                event["sources"] = [source_entry]
+                by_key[key] = event
+                merged.append(event)
+            else:
+                existing["sources"].append(source_entry)
 
     merged.sort(key=lambda e: e["start"])
     summary = f"{failures} of {len(sources)} calendars unavailable" if failures else ""